Meiosis
Name the 4 phases in meiosis
Prophase
Metaphase
Anaphase
Telophase
Define variation.
Variation is the differences in alleles for a gene. More variation is beneficial as there is a higher likelihood the individual will survive due to environmental change.
Define homozygous and heterozygous
Homozygous is when the 2 alleles for a gene are the same (TT or tt). Heterozygous is when the 2 alleles for a gene are different (Tt).
What is the normal dominance inheritance called?
Complete dominance.
What is evolution?
The change in characteristics over a long period of time?
Define natural selection
the process where beneficial phenotypes in a population are selected for, and unfavourable phenotypes are selected against.
What happens in metaphase?
The chromosomes line up in the centre of the cell.
Define mutation.
A permanent change in the base sequence of DNA. If it occurs in a gene, it could produce a beneficial, neutral or detrimental mutation.
Define genotype and phenotype
Genotype = the genetic formula of an individual e.g., Tt.
Phenotype = the expression of an allele e.g., brown eyes.
What are lethal alleles?
Alleles that produce a deadly phenotype.
What is a population?
A group of a single species living in a certain area.
Define artificial selection and explain the negative effects of this.
Humans selecting particular individuals to breed together.
Decreases genetic variation.
What 3 processes occur during meiosis to produce variation in the gametes?
1. Independent assortment
2. Crossing over
3. Segregation
What are the 4 main processes that produce variation in individuals (not gametes)?
1. Mutations
2. Sexual reproduction
3. Fertilisation
4. Environment
Define purebred
No variation in the characteristic being tested for. Either TT or tt.
Define incomplete dominance and give an example.
Incomplete dominance is seen in heterozygous individuals where the dominant and recessive allele combine to form a new phenotype e.g., red flowers (TT) x white flowers (tt) = pink flowers (Tt).
Define allele frequency
How often a particular allele is represented in a gene pool.
What is selected against in stabilising selection + give an example?
The two extremes - produces a narrower bell curve.
e.g., birth weight, eggs laid, coat colour.
How does crossing over produce variation?
Sections of DNA are exchanged between chromosomes. This can break up linked genes, therefore, producing variation in the gametes produced.
Compare and contrast haploid and diploid
Compare: They both contain chromosomes and the same number of types of chromosomes.
Contrast: Haploid contains 1 set of chromosomes whilst diploid contains 2 sets of chromosomes.
Why would you do a test cross?
How is a test cross conducted?
Why = to determine the genotype of an individual who has a dominant phenotype (either TT or Tt).
How = mate that individual with a homozygous recessive individual. If all offspring are dominant, parent must have been TT. If half of offspring are recessive, parent must have been Tt.
What phenotypic ratio (with a monohybrid cross) is seen in lethal allele inheritance?
2:1
What is the benefit to increased genetic diversity?
Increased chance of survival if/when there is environmental change.
What is selected against in directional selection + give an example?
One extreme - one side of the bell curve.
e.g., moths in industrial revolution, finch beaks.
How does independent assortment produce variation?
During metaphase, the homologous chromosomes line up randomly (but still with their correct chromosome). As there is 23 chromosomes in humans, this leads to 8 million different combinations of gametes produced.
Where does mutations need to occur to be passed onto offspring?
In the gametes. To see a change in phenotype, must occur in a gene.
Define dihybrid cross.
What is the normal phenotypic ratio for a dihybrid cross when 2 heterozygous individuals are mated together.
A cross between two individuals studying the inheritance of 2 genes, so a total of 4 alleles.
Phenotypic ratio = 9:3:3:1
Define co-dominance and give an example.
Co-dominance = When a gene has 2 dominant alleles and they are expressed at the same time.
e.g., red flowers (TT) x white flowers (WW) = red and white flowers (TW).
What is gene flow?
The movement of genes between different populations.
Increases genetic diversity within populations.
Decreases genetic diversity between populations.
Define and describe the two types of genetic drift
1. Founder effect
•when a small number of individuals leaves a population / becomes isolated from main population.
•New population may not carry all the same alleles as the parent population.
•New population will have a different gene pool.
2. Bottleneck effect
•When a large population is reduced in numbers due to a specific event. e.g., human action, climate change, or a catastrophic event (flood etc).
•The smaller population is not representative of the original populations’ gene pool. i.e., some alleles may be lost.